Using cycles volumetrics to make planet's atmosphere
I don't really know how the gradient texture node works so I usually make my own gradient texture using the generated coordinates and a Seperate XYZ node.

Here is what I have come up with for a homemade 3-dimensional linear falloff spherical gradient texture (plugged into a volumetric emission shader to visualize).


The object this material is applied to is actually a cube.

Here is the node setup:

Click to enlarge

It works by calculating the distance from every point to the point (0.5, 0.5, 0.5) in the object's bounding box (using generated coordinates).  This creates a gradient from 0 to 0.5 (going out from the center of the bounding box), but a gradient from 1 to 0 would be much more useful so this value gets multiplied by 2 and inverted by subtracting from 1.  (Ignore the math node labeled "clamp to [0,1]", it's accidentally left over from an earlier version.)

You can then plug this value into an RGB curves node (or through a few math nodes if you want to be more precise) to create whatever kind of falloff you want.

Here is the .blend so you don't have to manually copy all those math nodes!